Many techniques for gas purification and purity improvement have been proposed for various gases. For purification of carbon dioxide (CO2), diversified purification techniques for the respective purposes have been available ranging from such common techniques as recover and purify gas generated from an oil purification process to methods enabling high purification and intended for definite applications, for example, a semiconductor device manufacturing process.
With the recent rapid growth in semiconductor markets and the like, the following methods have been proposed: precision cleaning of optical components or microdevices using high purity liquefied CO2, and cleaning and drying of semiconductor wafers using supercritical CO2. Thus, applications of and demands for liquefied CO2 have been increasing. Furthermore, these applications need liquefied CO2 at a high quality level (for example, liquefied CO2 with a very small amount of impurities).
A method for producing carbon dioxide which method provides an ever higher purification level is needed in order to obtain CO2 with a high purity enough for use in a fine, nano-level, and precise process such as a semiconductor device manufacturing process. Furthermore, a process control method, a supply method, or an analysis technique is desired which allows such CO2 to be stably supplied.
A liquefied carbon dioxide supply system of a recirculating purification type is an apparatus that stably supplies high-purity liquid CO2 to a point-of-use. The liquefied carbon dioxide supply system of the recirculating purification type improves the purity of CO2 by using a recirculating system that vaporizes liquid CO2 into CO2 gas and which condenses the CO2 gas back into liquid CO2, to circulate CO2 through the recirculating system. Patent Literature 1 discloses a liquefied carbon dioxide supply system of a recirculating purification type. FIG. 1 illustrates an example of a configuration of a conventional liquefied carbon dioxide producing apparatus of a recirculating purification type which configured based on the description in JP-A-2006-326429 (Patent Literature 1).
A liquefied carbon dioxide producing apparatus shown in FIG. 1 includes: storage tank 11 that temporarily stores high-purity liquid CO2; pump 12 provided at an outlet of storage tank 11 to pump the liquid CO2; and filter 13 provided at an outlet of pump 12. Part of the liquid CO2 flowing out from filter 13 is supplied to point-of-use 30 in a branching manner, with the remaining liquid CO2 fed to cooler 15 via pressure regulating valve 14. Pressure regulating valve 14 is provided to regulate the pressure of liquid CO2 to be supplied to point-of-use 30 to a specified pressure value. Liquid CO2 supplied to and cooled by cooler 15 is then supplied to evaporator 16 for gas-liquid separation. Evaporator 16 incorporates a heater to form a gas-liquid interface of CO2 therein. The CO2 in a liquid state supplied to evaporator 16 is vaporized with refractory particles (particulates) remaining on a liquid phase side. The CO2 gas purified by vaporization in evaporator 16 is fed from a gaseous phase side outlet of evaporator 16 to condenser 18 via filter 17 for further removing particles. The CO2 gas is then cooled and liquefied again by condenser 18, and returned to storage tank 11 as liquid CO2. In this arrangement, CO2 recirculating system 10 comprises storage tank 11, pump 12, filter 13, pressure regulating valve 14, cooler 15, evaporator 16, filter 17, and condenser 18. A recirculating treatment is carried out on liquid CO2 not used at point-of-use 30. Particles having shifted to the liquid phase in evaporator 16 are discharged (blown out) to the exterior by opening valve 21 provided at a liquid phase side outlet of evaporator 16. Filter 13 is provided because pump might generate dusts.
Introduction device 20 that supplies CO2 to recirculating system 10 uses a liquefied carbon dioxide cylinder or a cold evaporator (CE) as liquid CO2 source 23 to function CO2 introduction means. Introduction device 20 includes vaporizer 24 that vaporizes liquid CO2 from liquid CO2 source 23, and filter 25 that removes particles from the CO2 gas vaporized by vaporizer 24. Introducing the CO2 gas having passed through filter 25 into condenser 18 allows CO2 to be supplied to recirculating system 10.
CO2 from a commonly available CO2 source contains not a small amount of particles whether the CO2 is in a gaseous state or a liquid state. Liquid CO2 commonly distributed for industrial use contains a particularly large amount of particles. When CO2 used for cleaning, drying, or the like is recovered and purified, the recovered CO2 contains a large amount of particles. Since such a CO2 source is used, the particles are removed by passing gaseous CO2 flowing out from vaporizer 24 through filter 25. At this time, the particles attach to and accumulate on an inner surface of vaporizer 24. However, the particles cannot completely be removed only by vaporizer 24 and filter 25, and the particles may enter recirculating system 10 through introduction device 20 together with CO2. In recirculating system 10, evaporator 16 and filters 13 and 17 remove particles having failed to be removed by introduction device 20 and particles generated in pump 12 and piping. Thus, the method for producing liquefied carbon dioxide according to Patent Literature 1 can prevent particles from being mixed into high-purity liquid CO2 thus purified.
JP-A-2006-347842 (Patent Literature 2) discloses a method for producing high-purity liquid CO2, the method including: vaporizing liquid CO2 in a vaporizer; passing the vaporized CO2 through a dehumidifier and an activated carbon filter, and then purifying and liquefying the resultant CO2 in a rectifier again to obtain liquid CO2 with an improved purity. According to this method, high-purity liquid CO2 is produced off-site and filled into a high-pressure container such as a cylinder, and the high-pressure container is transported to a point-of-use, where the high-purity liquid CO2 is taken out of the high-pressure container for use. Thus, the method described in Patent Literature 2 needs a high-pressure container dedicated to high-purity liquid CO2 and needs to handle such high-purity CO2 separately from normal-purity CO2. This makes management complicated and expensive.